Hydraulic fracturing is a common stimulation technique used to enhance production of fluids from subterranean formations. Hydraulic fracturing is typically employed to stimulate wells which produce from low permeability formations. In such wells, recovery efficiency is typically limited by the flow mechanisms associated with a low permeability formation.
During hydraulic fracturing, a viscosified fracturing fluid is pumped at high pressures and at high rates into a wellbore to initiate and propagate a hydraulic fracture. Once the natural reservoir pressures are exceeded, the fluid induces a fracture in the formation and transports the proppant into the fracture. The fluid used to initiate and propagate the fracture is commonly known as the “pad”. The pad may contain a heavy density fine particulate, such as fine mesh sand, for fluid loss control, or larger grain sand to abrade perforations or near-wellbore tortuosity. Once the fracture is initiated, subsequent stages of viscosified fracturing fluid containing chemical agents such as breakers, and containing proppants are pumped into the created fracture. The fracture generally continues to grow during pumping and the proppant remains in the fracture in the form of a permeable “pack” that serves to “prop” the fracture open. Once the treatment is completed, the fracture closes onto the proppants which maintain the fracture open, providing a highly conductive pathway for hydrocarbons and/or other formation fluids to flow into the wellbore. The fracturing fluid ultimately “leaks off” into the surrounding formation. The treatment design generally requires the fracturing fluid to reach maximum viscosity as it enters the fracture which affects the fracture length and width.
Fracturing fluids, including those containing breakers, typically exhibit poor transport properties. High pumping rates are required in order to impart a sufficient velocity for placement of the proppant in the fracture. In such treatments, the proppant tends to settle, forming a ‘proppant bank’, as the linear slurry velocity falls as a function of the distance from the wellbore. This effect is further believed to result in reduced stimulation efficiency as the effective propped length is relatively short. In addition, much of the settled proppant is often below the productive interval.
The recovery of the fracturing fluid is accomplished by reducing the viscosity of the fluid to a low value such that it flows naturally from the formation under the influence of formation fluids and pressure. This viscosity reduction or conversion is referred to as “breaking”. Historically, the application of breaking fluids as fracturing fluids at elevated temperatures, i.e., above about 120–130° F., has been a compromise between maintaining proppant transport and achieving the desired fracture conductivity, measured in terms of effective propped fracture length. Conventional oxidative breakers react rapidly at elevated temperatures, potentially leading to catastrophic loss of proppant transport. Encapsulated oxidative breakers have experienced limited utility at elevated temperatures due to a tendency to release prematurely or to have been rendered ineffective through payload self-degradation prior to release.
Improvements in hydraulic fracturing techniques are required in order to increase the effective propped fracture length and thereby improve stimulation efficiency and well productivity.